A lighting assembly usually comprises a light source and a lighting lens, the purpose of which is to collect the light that is produced by the source, and to distribute it within a light beam with a beam form which suits the use of this light. For this purpose, the lighting lens must suit the light source, in particular the form of the light emission zone of this source. In addition, the characteristics of the light beam that is produced by the lighting lens must correspond to the optical field to be illuminated and to the content thereof. Thus the performance of a lighting lens comprises its collection efficiency, that is to say the proportion of the light energy that is emitted by the source and which produces useful illumination. Other characteristics are in particular the size of the optical field that is illuminated, the uniformity or distribution of the lighting in this field, the angle magnitude of the beam, etc.
Particular difficulties may concern the production of illuminations in which the light beam is inclined with respect to an object plane, for example a focusing plane of an image capture apparatus. Light beam inclined with respect to a plane means a spatial configuration in which a mean direction of the beam forms an angle that is non-zero and less than 90° to a direction perpendicular to the plane. The form of the beam must then be adapted according to this inclination so that the entire required part of the object plane is suitably illuminated.
Other criteria that concern a lighting lens are its compatibility with a light power value that is sought, compatibility with any device cooling the light source, its size, the ease of assembly with the light source and the rest of the device, and its cost price.
Particular applications of lighting assemblies are biometric measurement devices. Such devices may in particular produce skin print images, images of the face, iris, subcutaneous venous networks of a finger or palm, etc. At least some of these devices may further need the following requirements:                sufficiently uniform lighting for an optical field that may be wide and have a cross-section elongated in one direction;        sufficiently intense illumination, in particular when the object to be photographed may be movable;        lighting that keeps its qualities when the object is moved perpendicular to the object plane, in particular over a field depth range that is desired for the image capture apparatus; and        a lighting direction that may be oblique with respect to the object plane, in particular for a contactless skin print sensor, and also possibly for capturing a face image.        
Lighting lenses known before the present invention, which are suitable for light emitting diodes, each consists of a dioptric central lens part, typically a convergent or positive lens, and a peripheral lens part, such as a parabolic reflector. Such lighting lens constitutes a collimator, with a light collecting efficiency which is in general only around 50%. The light beam that is produced at the output has a parallel beam structure and is not well suited to illuminating evenly an optical field the dimensions of which are typically 80×30×45 mm3. Furthermore, because of the composite structure of the light beam that is produced by such a lighting lens, it is very difficult to confer divergence on this light beam without light and dark rings appearing in the useful space that is illuminated. In other words, such lenses with a mixed dioptric/catadioptric structure are not suitable for producing illumination that is both divergent and uniform.